Do I need two copies of MindFeud on the 3DS to play multiplayer

The Gyro aiming is optional and can be turned off if you see fit. As an alternative you can simply use the analogue stick to aim which ever weapon you might be wielding (Its used to aim the slingshot, bow, boom-a-rang, hookshot, etc).

However; as a personal preference I find the gyro-aiming considerably easier and much more responsive to use compared to the stick aiming.

Yes, what you want to do is possible. While all 3DS software is region-locked, Nintendo DS software is not. Because that region-locking isn't encoded into the DS software, you can play software from all regions on your 3DS.

You can even check out this youtube video where someone demonstrates a DS games from North America, Japan, and Europe on their North American 3DS.